The stability of a transmission network supplied with energy from a number of different energy sources is dependent, at least in part, on the phase difference of the voltages at each end of the transmission line and on the rate of change of the phase difference. In accordance with the prior art, this out-of-step condition was relayed by apparatus primarily designed for fault protection which measured the apparent impedance of the line. In the prior art, a first fault detecting impedance relay having a desired distance characteristic depending upon the line impedance to the balance point was used for fault protection.
Additionally, for out-of-step protection, a second impedance relay having a distance characteristic which over-reached the balance point of the first relay and a timing device were added. In the case of a voltage frequency swing, the locus of the apparent impedance of the line would cross the characteristic impedance line of the second relay. If the swing were great enough, the locus would cross the characteristic impedance line of the first or fault detecting relay within a predetermined time interval as determined by the timer. When this occurred, the line connecting breaker would be actuated to disconnect the line from its power supplying bus.
If there were really an unstable condition, this disconnection would be desirable. However, in many instances this swing would not represent an unstable condition, and if the breaker was not tripped, the locus of the apparent impedance would pass outwardly through the impedance characteristics of the relays and the system would function normally. Under these conditions, the prior art system would result in an undesirable tripping of the breaker.
In accordance with this invention, the operation of an out-of-step relay is based entirely on the swing conditions and is not responsive to any fault or load conditions of the line. This is accomplished by providing first and second input signals; the length of which depend upon the phase relationship of the local and remote voltages. The difference in time length of these two signals is a measure of the rate-of-change of frequency or phase, while the sum of the two signals is a measure of the average phase difference between the local and remote voltages and the time length of the last of these two signals is a measure of the phase difference at the end of the cycle.